Is it permissible to post a video clip with background music in some forums, with a disclaimer to that effect, or with a request to lower the volume?

It is not permissible for website owners or visitors to publish any video or audio clip that contains background music, as this assists in spreading sin. Alerting visitors is not sufficient because the publisher causes them to sin. Publishing such clips is considered "bringing evil upon a Muslim." It has been narrated in a Hadith that Abu Bakr (may Allah be pleased with him) asked the Prophet (peace be upon him) to teach him what to say when he wakes up in the morning and when evening comes. He said: "O Allah, Creator of the heavens and the earth, Knower of the unseen and the seen, there is no god but You, Lord and Master of everything. I seek refuge in You from the evil of my soul, and from the evil of Satan and his polytheism (or his traps), and from committing evil against myself or bringing it upon a Muslim."